Existing ballast circuits of the type used with fluorescent and, in some cases, neon lamps suffer from certain technical problems. Particularly with respect to single-pin lamps in DC-powered mobile applications, a single interconnection carries both DC input current and AC output current. This causes AC noise and ripple to feed back into the DC power buss distribution system, which, in turn, can affect performance and operation of other systems connected to the DC buss.
FIG. 1 is a diagram of an existing single-pin lamp system. A 24-volt DC supply is connected to a ballast 102 and one side of the lamp 110. Typical of such systems, a safety switch 112 is provided to remove the DC power from the ballast if the lamp is removed. The connections are on either side of the pin, such that when the lamp is removed, the circuit is broken. The other pin 114 of the lamp 110 is connected to the ballast through line 116.
The configuration just described establishes two current paths, including a first current path 120 which includes noisy and high-frequency lamp current, and a second DC ballast input current path 122 which should not include any noise. However, due to the use of the safety switch 112, a single line 130 is common to both the lamp current and DC input current paths. As a result, noise or ripple may be coupled into the DC path, causing AC noise to infiltrate the DC power bus distribution system, leading to performance degradation and failure of other equipment powered by the DC buss, in some cases.
Given that it is not technically straightforward to filter out the conducted electromagnetic interference (EMI) prevalent in existing designs, the need remains for an improved filtering and control circuit configuration.